A. Field of the Invention.
The present invention is related to processes for the treatment of sludge coming from waste water treatment plants, and more particularly to a process in which the sludge produced by waste water treatment plant thickeners containing an average 2% solid content are immediately disinfected using as bioicides: heat and mineral acids which are subsequently neutralized by adding a calcium hydroxide slurry enriched with magnesium.
B. Description of the Related Art.
The disposal of organic waste such as sludge produced by municipal waste water treatment plants on arable lands and aquiferous layers, is subject to strict sanitary regulations, not only due to its high content of pathogen microorganisms and germs which represent a serious health risk, but for its high organic and nutrient content that propitiates disorganized biological growths which consume high amounts of oxygen and causes the death of most living beings existing in the medium in which the organic waste was discharged.
There are several well defined processes for treating sludge produced by waste water treatment plants that accomplishes all ecological rules and governmental regulations. All these processes have a common centrifugation or filtration step before the sterilization step is carried out, which raises the sludge solid content from 2.0% to an average 20%.
The disposal of the sludge is actually carried out using the following processes:
a) Land Fill.
This process consists on confining a filtered sludge having a solid content of 20% in a field having the following characteristics: a low permeability, absence of geological faults, very low volumes of rainfall, and located far away from towns and cities in order to avoid the pollution of auriferous layers with possible leaks of polluted liquid from the land fill. In US it is not allowed to dispose liquid sludge having a solid content of 2.0% without carrying out sterilization with lime or chloride.
Obviously, before the disposal of the sludge in the field, the surface of the field must be sealed with a waterproof and flexible plastic film, and after the deposit of each sludge layer, it has to be applied a very thick soil layer.
Among some of the disadvantages of the above referred process are: its high investment cost, its high operation cost and its null economic profit.
b) Sludge Drying Beds.
Drying beds are the most widely used method of sludge dewatering in the United States. Sludge drying beds are typically used to dewater filtered sludge having a solid content of 20%. After drying, the sludge is removed and either disposed of in a land fill. The most commonly used type of dry bed is the sand drying beds, which are generally used for servicing small and medium-sized communities. In a typical sand drying bed, the sludge must be placed on the bed at a depth no greater than 60 cm in order to guarantee the evaporation of the water. Although the principal advantage of the drying bed is its low cost, it is highly pollutant to the environment and the dry product cannot be used due to its high content of polluting agents such as bacteria and viruses.
c) Compost
This process comprises the steps of mixing a filtered sludge having a solid content of 20% with pasture and exposing the mix to solar heat in order to provoke a biologic decomposition or fermentation of organic matter, by which the temperature of the mass is raised to between about 70 to 75° C., thus destroying pathogen microorganisms and obtaining a product whose use is restricted to improve the quality of soil with organic matter due to its very low content (near 2.5% in dry basis) of major nutrients: nitrogen, phosphorous and potassium. Furthermore, when the compost is added to the soil, it is necessary to add chemical fertilizers in order to avoid affecting the quality of the soil.
This process has the disadvantage of using high volumes of pasture and having high transportation costs derived form transporting and distributing a very humid product and of a very low nutrient content.
d) Pasteurization.
This process comprises the steps of mixing a filtered sludge having a solid content of 20% with pasture; heating the resultant mix with steam and fast cooling the heated mix in order to eliminate all the pathogen microorganisms contained in the sludge and obtain a fertilizer called Compost free of any bacteriological polluting agent.
However, this process is recommended for using in low capacity wastewater treatment plants, in order to apply the produced Compost to arable lands in the proximity of the treatment plant, due to the high transportation costs and low selling price of the Compost, which make unprofitable the transportation of the Compost to places very far away from the plant.
c) Rotary Dryers or Fluidizing Bed Drying.
These processes palletize and reduce the water content of the sludge from 80 to less of 5% by using large amounts of external energy and using high priced fuels such as natural gas or fuel oil.
These processes have the advantage of offering the farmer a dry and non perishable product having a low water content and thus low transportation costs.
However, these processes have the disadvantage of increasing the production costs due to the high consume of energy, because if for each 1b of dry sludge, there are consumed 8,000 BTU, then, for example, if the process starts with a filtered sludge having a solid content of 20%, then 4 tons of water must be evaporated for producing 1 ton of dry sludge, which consume 445 liters of fuel oil, equal to 500 m3 of natural gas.
It is important to note that if the price of natural gas is $4.00 USD/MBTU, then 1 ton of dry sludge would cost $71.68 USD including only the primary energy expenses, and if the costs of other elements are included such as electricity, work force, materials, maintenance parts and other direct and indirect costs, the total product cost would be as much as $126.50 USD/ton, which is higher than the price of a chemical fertilizer such as ammonium sulfate, priced at $85.00 USD/ton granulated and having a nitrogen content of 21% compared with the major nutrient content of the dry sludge of 6.0% maximum.
d) Lime Stabilization.
In the lime stabilization process, lime is added to sludge having a solid content of 20% in sufficient quantity to raise the pH to 12 or higher. The pH creates an environment that is not adequate for the survival of microorganisms. Consequently, the sludge will not putrefy or create odors so long the pH is maintained at this level.
This process is becoming popular in US due to its low inversion cost, but for this process to operate economically, it has to be subsided, because generally, the sludge producer—the municipality—pays the treatment company near $160.00 USD by each ton of dry sludge in order to cover the expenses of the treatment company.
In this process, the treatment company receives the sludge as received from the water treatment plant thickeners, that is, with a solid content of 2.0%, therefore, the treatment company must invoice the treatment plant for the sludge pumping, filtering etc. and at the same time it must generate some profit so that it allows the treatment company to recover the inversion costs during the time it serves to the municipality.
Finally, lime stabilization does not destroy the organics necessary for bacterial growth. Therefore the sludge can not be used and must be treated with an excess of lime or disposed of before the pH drops significantly.
e) Sludge Incineration.
The sludge incineration process, has some advantages when compared with the sludge drying process, due to the fact that during the incineration it is used the combustion heat of the organic matter contained in the sludge, thus decreasing the energy consumption and furthermore, the air volumes to be heated and moved are significantly smaller which constitute an additional energy consumption decrease.
The main disadvantages of the sludge incineration process are related with the drying of the sludge, mainly because of the following problems: the investment, operation and maintenance costs of the incineration step are higher than the costs of the drying step, furthermore, the incineration process is highly pollutant to the environment, because the gaseous effluents produced by the incinerator are very acidic, and contain very toxic metallic compounds such as lead, arsenic, cadmium and mercury (normally present in the urban wastewater), which are considered as very dangerous for the environment and for the health of humans and animals. Additionally, the disposal of the ashes generated by the incinerator and recollected in the ash separator, have the same polluting agents as the gaseous effluents and therefore the use of said ashes for the production of cement for example, is very restrained.
f) Other Processes.
There are other processes for the treatment of sludge and animal dry excrement which add NPK major nutrients and uses the same raw materials as the fertilizer industry, and sometimes, the reaction heat for heating the mass and evaporating the water contained in the sludge, specifically the process of the U.S. Pat. No. 6,159,263, in which the untreated sludge having a solid content of 20% are diluted with 30% of water, and when liquefied, are pumped to a tubular reactor working at 75 p.s.i in which are feed phosphoric acid, sulfuric acid and ammonia, which react and generate sufficient heat to raise the temperature of the mass to 150° C.
The mass exiting the tubular reactor is discharged to a rotary granulator working at atmospheric pressure, by which part of the water is evaporated by flashing when the pressure is reduced.
Subsequently the mass is dried using external energy, requiring the equivalent of 15 million BTU by each ton of dry sludge, therefore, this process consume the 85% of the energy that a common sludge drying process would consume.
U.S. Pat. No. 5,411,586 discloses a process in which hen drop is granulated with the aid of formaldehyde-urea diluted resins and finally drying the produced granules using external energy. This process has a high energy consume and therefore it does not have any advantage compared with the above referred processes.
U.S. Pat. No. 4,038,180 discloses a process in which the filtered sludge having a solid content of 20%, is mixed with acids or mineral anhydrides such as phosphoric or sulfuric, and the acid mix is feed to a fertilizer granulation plant, in which the acids are neutralized with ammonia and based on the neutralization heat, it is evaporated an 85% of the water contained in the sludge.
This process has serious corrosion issues and problems controlling granulation, because there are not included the mechanisms needed for the generation of intermediate acid ammonium sulfate required before the final ammoniating and before its transformation into neutralized ammonium sulfate.
All ammonium sulfate granulation processes known such as TVA in US, INCITEC in Australia, Cominco and ESSO Chemicals in Alberta, Canada, use metallic additives derived from zinc and aluminum, and have the common inconvenient of having high fixed investment costs, due to high recycle volumes and to the specialized materials needed for avoiding corrosion, because the high working temperatures cause the plastics to corrode in presence of the diluted acids and the hot acid ammonium sulfate.
U.S. Pat. No. 4,081,366 discloses a process in which a formaldehyde-urea polymer is added in order to co-polymerize and separate organic matter contained in the sludge by filtration, when an olygomer having a high nitrogen content (21% dry basis) is formed, comprised by 50% of UREAFROM and 50% of sludge organic matter. The filtrated solids have a water content of approximately 80%, and consequently this process is very costly.
As it can be seen, there have been many attempts for achieving the best use of the sludge produced by water treatment plants, mainly due to the great importance of the resource.
Only in U.S. there were produced seven and a half million tons of dry sludge in 1998, product of processing more than 35,000 million wastewater gallons by day and 60% of the 7.5 million tons of dry sludge were recycled to arable lands for improving the quality of the soil. The EPA estimates that in the year 2010 there will be generated 8.2 million tons of dry sludge, 70% of which will be applied to arable lands, considering its high nutrient content.
Applicant developed a process in which the energy consume is reduced a 90% thanks to the addition of a special filtration step in which the water content is reduced a 50% and the remaining water is evaporated by the exothermic reaction occurring in the process.
Additionally, applicant's process include mechanisms that allow some of the by products generated by some of the reactions such as gypsum, to adsorb crystallization water and reduce the humidity of the mass without using external energy.